core: add 'DefaultRestrictSUIDSGID' config option (#38126)

closes #37602, see there for extra motivation and considered
alternatives.

On typical systems, only few services need to create SUID/SGID files.
This often is limited to the user explicitly setting suid/sgid, the
`systemd-tmpfiles*` services, and the package manager. Allowing a
default to globally restrict creation of suid/sgid files makes it easier
to apply this restriction precisely.

## testing done
- built on aarch64-linux and x86_64-linux
- ran a VM test on x86_64-linux, checking for:
    - VM system boots successfully
    - defaults apply (both `yes`, `no`, and undefined)
    - systemd tmpfiles can set suid/sgid on journal log path
- Other services explicitly defining `RestrictSUIDSGID=no` can create
suid files

diff --cc man/org.freedesktop.systemd1.xml
Simple merge

diff --cc man/systemd.exec.xml
Simple merge